FIG. 13 is a perspective view showing a construction of a conventional waveguide type polarizer shown in JP 11-330801 A, for example. In addition, FIG. 14 is a side view of a branch portion useful in explaining a distribution of an electric field of a basic mode when inputting a horizontally polarized wave in the waveguide type polarizer shown in FIG. 13. Moreover, FIG. 15 is a cross sectional view of a main waveguide useful in explaining a distribution of an electric field of an unnecessary higher mode generated when inputting a horizontally polarized wave in the waveguide polarizer shown in FIG. 13.
In FIGS. 13 to 15, reference numeral 31 designates a rectangular main waveguide through which a vertically polarized electric wave and a horizontally polarized electric wave are transmitted; reference symbols 32a and 32b respectively designate two rectangular branching waveguides branching perpendicularly and symmetrically with respect to a tube axis of the main waveguide 31; reference symbols 33a and 33b respectively designate metallic thin plates which are inserted into the main waveguide 61 and which each have arcuate cutouts symmetrically formed; reference symbol P1 designates an input terminal of the main waveguide 31; reference symbol P2 designates an output terminal of the main waveguide 31; reference symbols P3 and P4 respectively designate output terminals of the branching waveguides 32a and 32b; reference symbol H designates a horizontally polarized electric wave; and reference symbol V designates a vertically polarized electric wave.
Next, an operation will hereinbelow be described. For a basic mode (TE01-mode) of the horizontally polarized electric wave H inputted through the terminal P1 of the main waveguide 31, each of a space defined between an upper sidewall of the main waveguide 31 and the metallic thin plate 33a, a space defined between the metallic thin plates 33a and 33b, and a space defined between the metallic thin plate 33b and a lower sidewall of the main waveguide 31 is designed so as to be equal to or smaller than a half of a free-space wavelength of a frequency band in use. Thus, the horizontally polarized electric wave H hardly leaks to the terminal P2 side of the main waveguide 31 due to those cut-off effects.
In addition, since as shown in FIG. 14, arcuate cutouts are symmetrically formed in each of the metallic thin plates 33a and 33b, when inputting the horizontally polarized wave, an electric field is distributed in a state in which two rectangular waveguide E-plane arcuate bends excellent in reflection characteristics are equivalently placed in a branch portion into a symmetrical form. Thus, the horizontally polarized electric wave H of a basic mode inputted through the terminal P1 is efficiently outputted to the terminals P3 and P4 while suppressing a reflection to the terminal P1 and a leakage to the terminal P2.
Moreover, the two metallic thin plates 33a and 33b have the same shape, take a vertically symmetrical shape within the main waveguide 31 and are mounted in positions away from the vicinity of a center. Thus, as shown in FIG. 15, when inputting the horizontally polarized wave, the vertically symmetrical planes become magnetic walls in a region defined between the metallic thin plates 33a and 33b and hence, in principal, a TE20-mode as a higher mode causing a degradation of the reflection characteristics is not generated. As a result, an effect is obtained in that the degradation of the reflection characteristics when inputting the horizontally polarized wave can be suppressed to a frequency band with a frequency about twice as high as a cut-off frequency of a basic mode (TE01-mode) of the horizontally polarized wave H.
On the other hand, for a vertically polarized electric wave V of a basic mode (TE10-mode) inputted through the terminal P1 of the main waveguide 31, each of a sidewall space defined between surfaces each having a large width of the branching waveguide 32a and a sidewall space defined between surfaces each having a large width of the branching waveguide 32b is designed so as to be equal to or smaller than a half of the free-space wavelength of the frequency band in use. Thus, the vertically polarized electric wave hardly leaks to the sides of the terminal P3 and the terminal P4 of the branching waveguides 32a and 32b due to those cut-off effects.
In addition, the metallic thin plates 33a and 33b are mounted so that the plate surfaces thereof perpendicularly intersect a direction of an electric field of the vertically polarized wave V in the main waveguide 31, and also a thickness of each of the metallic thin plates 33a and 33b is designed so as to be much smaller than the free-space wavelength of the frequency band in use. For this reason, the electric wave V of the basic mode is hardly reflected by the metallic thin plates 33a and 33b. Therefore, the vertically polarized electric wave V of the basic mode inputted through the terminal P1 is efficiently outputted to the terminal P2 while suppressing the reflection to the terminal P1 and the leakage to the terminals P3 and P4.
The conventional waveguide type polarizer is constituted by: the rectangular main waveguide 31; the two rectangular branching waveguides 32a and 32b branching perpendicularly and symmetrically with respect to the tube axis of the main waveguide 31; and the metallic thin plates 32a and 32b inserted into the main waveguide 31. Then, the vertically polarized wave and the horizontally polarized wave which have entered through the input terminal P1 of the main waveguide 31 are outputted through the output terminal P2 of the main waveguide 31 and the output terminals P3 and P4 of the branching waveguides 32a and 32b, respectively. Thus, there arises a problem in that a miniaturization, and shortening of the axis are difficult to be made with respect to a direction of the tube axis of the main waveguide 31.
In addition, in general, in a frequency band in the vicinity of the cut-off frequencies of the basic modes (the TE10-mode and the TE01-mode) of the vertically polarized wave and the horizontally polarized wave, an abrupt change in frequency of a guide wavelength is observed, and along therewith, an abrupt change in frequency of discontinuity of an impedance in the branch portion of the rectangular waveguide 31 is also involved. Thus, in the conventional waveguide type polarizer, it is difficult to suppress the degradation of the reflection characteristics of both the polarized waves in a frequency band in the vicinity of the cut-off frequencies.
The present invention has been made in order to solve the problems as described above, and it is therefore an object of the present invention to obtain a waveguide type polarizer, which enables a miniaturization thereof, shortening of an axis, and broad band promotion, and which has high performance.